Extract url from next_image_link

The function below is derived from adjacent_image_link() function. You can get both previous and next image source by this function. It can be used inside or outside loop.

Uses

 // get next image src
 $src = wpse145194_get_adjacent_image_link();


 // get previous image src
 $src = wpse145194_get_adjacent_image_link(true);

Function

function wpse145194_get_adjacent_image_link( $prev = false, $size="thumbnail", $post_id = false) {

    if(!$post_id)
        $post = get_post();
    else
        $post = get_post($post_id);


    $attachments = array_values( get_children( array( 'post_parent' => $post->post_parent, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID' ) ) );


    foreach ( $attachments as $k => $attachment )
        if ( $attachment->ID == $post->ID )
            break;

    $k = $prev ? $k - 1 : $k + 1;

    $link = $attachment_id = null;

    if ( isset( $attachments[ $k ] ) ) {
        $attachment_id = $attachments[ $k ]->ID;
        // $link = wp_get_attachment_link( $attachment_id, $size, true, false, false );
        $link = get_attachment_link( $attachment_id);
    }

    return $link;
}

Code not tested